Electroless nickel palladium gold processes for wire bonding and flip chip soldering are known in the prior art and described, for example, in European patent application EP 0 701 281 A2. Similar processes are described in U.S. Pat. No. 6,445,069 and European patent application EP 1 126 519 A2. The under bump metallisation (UBM) is driven by technological aspects, such as wafer level miniaturisation, electrical signal integrity and metal stack reliability. Wafers are getting thinner and, thus, any applied metal stack can cause undesired warpage of the wafer.
According to the prior art, a Cu or Al layer is coated first on the wafer surface, followed by the plating of Ni—P and Pd (and optionally Au) on individual compartments of the wafer surface (“pads”). The layers of the prior art stacks have a deposit internal stress that results in a warpage of the wafer.